Would you say there is any risk for vendors or is it rare to have problems?
There is ALWAYS risk with ANY form of payment.
If you are accepting U.S. dollars, there is a risk that you will accidentally accept high quality counterfeit bills. You may not know this has happened for a day or so until you try to deposit the bill at the bank. By then, it may be difficult to recall many details about the person that gave you the counterfeit when you talk to the police and they attempt to track down the criminal.
If you are accepting credit cards, there is a risk that a chargeback may occur against your business for a variety of reasons. If it is due to credit card fraud, you may not know this has happened for several weeks (or even months). By then, it may be difficult to recall many details about the person that used the credit card when you talk to the police and they attempt to track down the criminal.
If you are accepting bitcoins, there is a risk that they will successfully get a replacement transaction confirmed before the transaction that you see is confirmed. You may not know that this has happened for an hour or so. Of the three situations we've discussed here, you have the best chance of recalling some details about the thief when you contact the police in this situation.
Furthermore you can create software that checks some very simple details that will
drastically reduce the risk when accepting bitcoin transactions.
If you verify that:
- Your wallet is well connected to multiple geographically distributed peers (or directly connected to several large mining pools)
- The transaction has been well propagated (especially if your software has heard about the transaction from several large mining pools)
- The transaction includes a reasonable fee per kilobyte
- All inputs to the transaction are already confirmed
Then you can be VERY confident that the transaction will confirm. It would be extremely difficult (nearly impossible) to reverse such a transaction. I'd personally feel comfortable accepting a transaction worth hundreds of dollars without confirmation if it met the criteria above.